Principles, Theory, History
"Activities of users at terminals and most application programs should remain unaffected when the internal representation of data is changed and even when some aspects of the external representation are changed." – E.F. Codd 1970
All information is represented by the same model.
Data consistency is enforced not by imperative application rules but by declarative database constraints.
true | falseThe mathematical formalism is useful for reasoning, but all database implementations deviate from the strict mathematical model for practical purposes.
GROUP BY AND ORDER BY)NULL)OUTER JOIN)